All types of rock and soil contain some level of uranium, the parent element, though mostly it is found only in tiny proportions, usually in the region of one to three parts per million. However, some types of rock have a far greater presence of uranium. Among them are volcanic rocks, granites, shale and rocks containing phosphate. These types and the soils that derive from them can have as much as 100 parts per million of uranium.
Because uranium is present in all rocks and soil, so are radium and, consequently, radon, which has the chemical symbol Rn. And because it is a gas, it has a greater tendency to move and discharge into the atmosphere. It does so by escaping through fractures in rocks and the spaces inherent in soil. However, it is a heavy gas, heavier than air, and therefore tends collect in low-lying areas.
Radon is also found in water, though it moves more slowly through water than through air. Typically, radon only travels less than 1 inch in water-saturated rocks or soils before it decays, but it can move as much as tens of feet through dry rocks or soils. The rate at which radon moves is determined by how much water is in the pore space, the percentage of pore space in the soil and the interconnectedness of the pore spaces.
Radon is toxic for humans exposed to it for lengthy periods. It is most often the cause of lung cancer because, as a gas, it can be inhaled. It was once the scourge of workers such as miners, though modern ventilation systems have much reduced the incidence of the disease in these types of operations. It has been estimated that around 20,000 people in the United States die each year as a result lung cancer caused by radon.